When did Dostum switch sides and join Gulbuddin Hekmatyar in the Afghan Civil War?

Dostum joined Najibullah after the Soviets left the area.

As for the unpredictable Uzbek commander Dostum, Rasheed said no one knew where he would stand. Dostum had fought the Soviets in the 1980s alongside the Mujahideen but had defected and joined Najibullah's communist puppet regime after the Soviets had left.